In certain optical transmission systems which require a relatively large number of serially-connected optical repeaters, each repeater constitutes a potential point of failure for the entire system. A common approach to safeguarding against system failure due to failure of a single repeater is to use ultra-high reliability components. These can be extremely costly, however, and by themselves do not provide for a failure-mode recovery.
Some digital optical transmission systems, such as underwater surveillance systems, have relatively close repeater spacing as compared to long-haul systems where as few repeaters as possible are used. With close repeater spacing, there often is ample optical signal strength entering each successive repeater to add an optical repeater bypass path around each repeater to provide some tolerance to repeater fault.
While repeater bypasses are generally known in the art, it is not always easy to implement a bypass. One problem is that the bypass signal output must not interfere with the next-in-line digital repeater's ability to accurately lock onto and amplify the bit stream. At the same time, the bypass signal output must be great enough that the next-repeater can detect the bit stream to be amplified. A further problem relates to detecting what type of malfunction has occurred, and whether it necessitates the shutting down of the local repeater.